How Do I Determine that Implant Site is Clean from a Previous Infection?

Anon, a prosthodontist, asks:

I have a patient who had a previous chronic periapical infection on 31 (lower right second molar). He had persistent pain and fistula with pus coming out on finger palpation.

The 31 was then removed with antibiotic prescriptoin (augmentin) and curreted to remove the granulation tissue and irrigated and the patient had 2 implants placed on 31 and 30 after the site healed 4 months later. At the time of implant placement, patient was assessed for any symptoms: no pain,no pus on palpation and no fistula. The bone and gingiva seemed healthy. A periapical radiograph was also taken, and the site seemed clean.

After two months of implant placement, the patient came for a regular check up, and I found there was pus coming out from implant 31 on light finger palpation (the implant was remained uncover and untouched). The patient had no pain or any other symptoms after the implant placement. The healing was uneventful.

I have also noted from previous posts, that one of the common reason for implant failure is previous tooth infection.

I was just wondering what are the points that you will examine to determine that the site is ‘free and clean from any previous infection’? And how should we clean the extraction site at the time of extraction without jeopardizing the remaining thin wall bone?

thx for the comments.